An electric power steering device applying a steering assist force to a steering apparatus of a motor vehicle on the basis of a rotating force of a motor is constructed such as to apply the steering assist force to a steering shaft or a rack shaft by a transmission mechanism such as a gear or a belt or the like via a reduction gear on the basis of a driving force of the motor.
An example of construction of the electric power steering device mentioned above is shown in FIG. 1, and is described. A shaft 102 of a steering wheel 101 is coupled to a tie rod 106 of a running wheel via a reduction gear 103, universal joints 104a and 104b and a pinion rack mechanism 105. The shaft 102 is provided with a torque sensor 107 detecting a steering torque of the steering wheel 101, and a motor 108 assisting a steering force of the steering wheel 101 is coupled to the shaft 102 via the reduction gear 103. Further, a motor control of the electric power steering device is executed by a control unit 109 by setting a torque value detected by the torque sensor 107, a vehicle speed detected from a vehicle speed sensor (not shown), a rotation angle of the motor detected by a Hall sensor 110 or the like, and the like to input values. The control unit 109 is mainly constituted by a CPU, and a motor control is executed in an inner portion of the CPU in accordance with a program.
One example of a control system controlling the motor 108 of the electric power steering device mentioned above is shown in FIG. 2 and is described. In FIG. 2, a current command value calculating unit 120 calculates a current command value Iref by being input the torque value detected by the torque sensor 107, a difference from a detection value Im of the motor current is calculated by a subtraction unit 121, a duty ratio is determined by a current control unit 122, and a motor driving unit 123 executes a PWM control in accordance with the duty ratio so as to drive the motor 108.
In the electric power steering device mentioned above, on the assumption that the torque value detected by the torque sensor 107 is correctly detected, the control of the electric power steering device is executed. However, when the fault practically occurred in the torque sensor 107, and an abnormal torque detection value is input, there is a risk that an abnormal operation is caused with respect to the operation of steering wheel. Accordingly, various countermeasures have been conventionally employed.
For example, Japanese patent document 1 (Japanese Patent Application Laid-Open No. 2000-318633) employs a control method as shown in FIG. 3, and corresponds to an abnormality of the torque value output by the torque sensor. In other words, when the torque value output by the torque sensor becomes abnormal, and the abnormality continues for a predetermined time (tA), the control system interrupts a steering assist force command value corresponding to an output value of a current control calculation based on the torque value. In the case that the abnormality continues for a predetermined time (tB) which is longer than the predetermined time (tA), the control system interrupts a power source of motor driving.
FIG. 4 shows a relation between a torque value and a motor current in the case that the torque sensor causes an earth fault and the torque value corresponding to the output of the torque sensor becomes zero, in the control system mentioned above. In the case of the control system, since the steering assist command value is calculated on the basis of the output value of the torque sensor which becomes abnormal, for the predetermined time (tA), the steering assist force becomes abnormal, and the steering wheel performs a movement which is not intended by a driver. Further, if the abnormality continues for the predetermined time (tB) or more, the motor power source is interrupted. Accordingly, in the case that a great torque is applied to the steering wheel, the torque undesirably changes suddenly.
As the other countermeasure, there is a control system shown in Japanese patent document 2 (Japanese Patent Application Laid-Open No. 2000-329628). In accordance with the patent document 2, if a severe crucial abnormality such as a voltage drop, a short break or the like occurred in the power source of the torque sensor, a fail switch is disconnected, a torque value before the fail switch is disconnected is kept as the torque value corresponding to the output of the torque sensor, and the steering assist force command value is determined by multiplying the kept value by an output gain. Further, since the control system controls in such a manner as to gradually attenuate the subsequent steering assist force, the steering assist force does not change suddenly. FIG. 5 shows a relation between an output (a torque value) of the torque sensor and a motor current in the case that the torque sensor breaks down, for example, the torque sensor causes the earth fault, in the case of using the control system mentioned above. A negative maximum current flows through the motor until a determination of the fault is confirmed after the fault is caused, however, since the motor current is gradually reduced from the torque just before the fault is caused after the decision of the fault, the sudden torque change as described in the patent document 1 is not generated.
However, in the case that the earth fault is caused while the torque value causes a chattering as shown in FIG. 6, there is a possibility that a value shown by a black circle is sampled in the case of sampling the output value of the torque sensor by an AD converter. In the case mentioned above, the earth fault can not be detected until the sampling value shown by the black circle becomes equal to or less than a threshold value for detecting the earth fault, and the control is carried on as it is. Further, the motor current is gradually reduced from the torque value just before the fault is caused after the earth fault is detected. However, since the motor current is gradually reduced from the unstable torque value in which the chattering is caused, the reduction is undesirably started from an inverted torque to the torque just before the fault at the worst.
Further, in accordance with the method described in the patent document 2, since the abnormal current flows through the motor during the determining period until the determination of the fault is confirmed after the fault is caused, it is impossible to make the determining period of the fault determination too long. However, in order to prevent the fault detection from being erroneously executed so as to ensure the fault detection, it is antagonistically desirable to make the fault determination confirming time as long as possible, and a problem occurred.
There is a problem that the conventional control system controls on the basis of the abnormal torque value until the fault of the torque sensor is confirmed in the case that the torque value corresponding to the output of the torque sensor becomes abnormal, and the steering assist force becomes abnormal.
Further, even in the case that the substitute torque value is used in place of the abnormal torque value, there is a problem that an appropriate substitute torque value is not calculated. As a result, if the output of the torque sensor becomes abnormal, the steering wheel executes the movement which is not intended by the driver, so that there is a problem that an uncomfortable feeling of the operation of steering wheel is applied to the driver.
Further, since the abnormal current flows through the motor during the determining period until the determination of the fault is confirmed after the fault is caused, it is impossible to elongate the confirming time of the fault determination, so that there is a problem that an erroneous detection about the fault detection of the torque sensor is generated. Particularly, in the chattering phenomenon generated in the case that a wiring transmitting the torque value detected by the torque sensor is disconnected, there is a problem that it is hard to correctly control the torque of the electric power steering device.
The present invention is made by taking the circumstances mentioned above into consideration, and a first object of the present invention is to provide a controller of an electric power steering device which does not apply an uncomfortable feeling to a operation of steering wheel while securing a fault determination confirming period enough long to prevent an erroneous detection of a torque sensor fault, and can secure a safety operation of steering wheel, even in the case that an output value of a torque sensor becomes abnormal.
Further, a second object of the present invention is to provide a controller of an electric power steering device which can safely switch an output value of a torque sensor and an substitute torque value used at an abnormal time even in the case that a plurality of torque abnormality detecting means are provided.
Further, a third object of the present invention is to provide a controller of an electric power steering device which can precisely estimate an substitute torque value used for a control during an abnormal period of the torque sensor.